Rightsizing Availability

To plan availability of systems and applications, assess the availability
needs of the user groups that access different applications. For example,
external fee-paying users and business partners often have higher quality
of service (QoS) expectations than internal users. Thus, it may be more acceptable
to internal users for an application feature, application, or server to be
unavailable than it would be for paying external customers.

The following figure illustrates the increasing cost and complexity
of mitigating against decreasingly probable events. At one end of the continuum,
a simple load-balanced cluster can tolerate localized application, middleware,
and hardware failures. At the other end of the scale, geographically distinct
clusters can mitigate against major catastrophes affecting the entire data
center.

To realize a good return on investment, it often makes sense identify
availability requirements of features within an application. For example,
it may not be acceptable for an insurance quotation system to be unavailable
(potentially turning away new business), but brief unavailability of the account
management function (where existing customers can view their current coverage)
is unlikely to turn away existing customers.